This is a very important reason why books, in general, contain better information that websites. On websites, people care a lot less about the correctness of the information. You can just update stuff later (of course, this doesn't always happen). Also, sites are a very volatile medium. I often bookmark pages with interesting information to read later, and it inevitably happens once in a while that a site went down a…
I've recently had this problem with some online fiction that I had bookmarked. Now, I was able to recover thanks to the Wayback Machine, but I really shouldn't depend on that.
I should really put some thought into archiving pages I like or getting a Pinboard account.